Ground pressure and convergence for TBM driven tunnels in visco-plastic rocks

The time-dependent behaviour of rocks may be the cause of difficult tunneling conditions, especially for Tunnel Boring Machine (TBM) driven tunnels at great depth. In fact, the large deviatoric stresses associated with the excavation may induce squeezing conditions that, in the most severe cases, show up in a short time, during the tunnel excavation stages. In this case, the creep deformations lead to large tunnel convergence and consequent high pressures exerted on the permanent liner and on the TBM itself, in particular the cutter head and the shield. The possible occurrence of a TBM jamming should be predicted in advance, so that suitable countermeasures can be taken to prevent it. The increase of the ground pressure on shield and permanent liner during the tunnel face advance is discussed here on the basis of some results from a series of finite element analyses, carried out in axisymmetric regime and adopting an elasto-visco-plastic rheological model. Some comments are drawn on the relevance of some parameters, such as the tunnel advance rate and the tunnel section overboring, for the correct prediction of the ground pressure and of the risk of the machine jamming.